The present invention relates to a multibeam scanning device for dynamically deflecting a plurality of beams to simultaneously scan on corresponding target surfaces.
Various types of multibeam scanning devices for simultaneously scanning a plurality of optical beams on corresponding scan target surfaces respectively (the so-called “tandem multibeam scanning devices”) have been proposed and brought into practical use today. Such tandem multibeam scanning devices are installed in a wide range of image formation devices such as laser printers, copy machines and facsimile machines.
In such devices, electrostatic latent images corresponding to particular colors are formed on a plurality of scan target surfaces (surfaces of photoconductive drums, etc.) respectively by the multibeam scanning device, which simultaneously scans a plurality of optical beams on the scan target surfaces, respectively. The electrostatic latent images formed on the scan target surfaces are developed by a plurality of developing units for the colors, respectively, and the developed images of the colors are successively transferred to a print medium (paper, etc.) and superposed on one another, by which a color image is formed on the print medium.
The multibeam scanning device typically includes an fθ lens having fθ characteristics, a plurality of mirrors placed on optical paths of the optical beams for deflecting the optical paths, a plurality of cylindrical lenses placed on the optical paths having refractive power for converging the optical beam (laser beam) in the auxiliary scanning direction while correcting various aberrations (e.g., fθ characteristics errors and field curvature (curvature of field, image surface curvature) in the main scanning direction), etc. There is a tendency in recent years toward the reduction of the number of optical elements for reducing costs, and multibeam scanning devices using only one deflecting system (e.g. polygon mirror) and only one fθ lens, to share each component among the colors, have become popular.
In such multibeam scanning devices, a plurality of optical beams are incident upon a reflecting surface of the polygon mirror at different incident angles in the auxiliary scanning direction. The optical beams dynamically deflected by the reflecting surface of the polygon mirror then pass through the fθ lens at different positions in the auxiliary scanning direction. In this case, there exists an optical beam that travels apart from the optical axis of the fθ lens (as an optical system in the device). The misalignment of the optical beam from the optical axis of the optical system causes a scan line formed on the scan target surface by the optical beam traveling through the optical system (apart from the optical axis) to become a curved or distorted line, which is called “bow,” due to distortion caused by the optical system.
Since the optical beams travel at different image heights in the fθ lens, the degree of distortion of the bow differs among the optical beams. If the “bows” of the scan lines occur, each image formed on the scan target surface is distorted and thereby image quality is deteriorated. The image quality is further deteriorated in the color superposition since the images of the colors are not superposed on one another precisely (misregistration). To avoid the problems, the bow on the scan target surface is corrected generally by placing each cylindrical lens (placed on the image side of the fθ lens) eccentrically, that is, by slightly shifting the position of each cylindrical lens in the auxiliary scanning direction relative to the incident optical beam.
In the formation of a color image by such a tandem multibeam scanning device, there still exists a phenomenon other than the bow that deteriorates the image quality: a shift of scan lines in the auxiliary scanning direction. If the shift of scan lines in the auxiliary scanning direction occurs, image quality is deteriorated by the shift of each image formed on each scan target surface and then further deteriorated by the misregistration occurring in the color superposition. In order to avoid the problems, the shift of scan lines in the auxiliary scanning direction is corrected generally by adjusting the angle of each mirror placed between the fθ lens and each cylindrical lens.
However, if the angle of the mirror is adjusted depending on the shift of the scan line occurring in the auxiliary scanning direction, the optical path of the optical beam is changed and thereby the bow revives on the scan target surface. Therefore, in conventional techniques, the bow was corrected by changing curvature characteristics of the cylindrical lens by deforming and curving the cylindrical lens. An example of such a configuration is disclosed in Japanese Patent Provisional Publication No. HEI10-268217 (pages 2-4, FIGS. 1, 5 and 6).
However, such adjustment deforming and curving the cylindrical lens is a complex and troublesome task for factory workers and the workers are required to be trained and skillful. Further, deforming the optical element by external force might deteriorate optical characteristics of the optical element itself. Such a method of deforming each cylindrical lens increases the number of necessary steps in the manufacturing process, elongates assembly time of the multibeam scanning device, drives up the manufacturing costs, and causes deterioration of images formed by the device.